Directional couplers have been used in various applications. FIG. 17 shows an exemplary application of a directional coupler, specifically, a typical system in a cellular phone unit which includes a power amplifier (or PA), a directional coupler, and a wave detecting circuit. The reference symbol ANT denotes an antenna. The system shown in FIG. 17 is adapted to monitor power through the directional coupler. In this system, the forward power from the PA can be accurately monitored by greatly reducing the influence of the mismatch in impedance at the antenna terminal on the detected voltage by virtue of the directivity of the directional coupler. Such monitoring systems are often used in GSM (Global System for Mobile Communications) terminals, which are widely used overseas, and in CDMA terminals. However, they are not limited to use in terminals, but are the most common systems for monitoring transmission power.
FIG. 18 shows an exemplary relationship between the directivity of the directional coupler and the error in the power measurement. Specifically, FIG. 18 shows, as a function of the directivity of the directional coupler, the calculated error in the forward power measurement obtained from the signal power appearing at the coupled port CPL when the system is operated under the mismatched load condition that VSWR=4:1. When a main signal is transmitted from the input port IN to the output port OUT through the directional coupler, part of the forward power can be coupled out to the coupled port CPL to monitor this forward power, or transmission power. FIG. 18 indicates that in order to achieve a measurement error of less than 0.5 dB, the directional coupler must have a directivity of more than 20 dB.
The degree to which the input port IN is coupled to the coupled port is referred to as “coupling” or “coupling factor.” That is, the coupling is the ratio of the CPL signal power to the IN signal power and is typically approximately −10 dB to −20 dB.
A mismatch in impedance at the output port OUT results in some reflection of the signal at that port, so that a reflected wave travels back from the output port OUT to the input port IN. At that time, part of the input wave (reflected wave) to the output port OUT is absorbed in the isolated resistance. (That is, the relationship between the output port OUT and the isolated port is similar to that between the input port IN and the coupled port CPL.) A reflected wave component also appears at the coupled port. The term “isolation” as used in the art refers to the ratio of the signal power appearing at the coupled port to the signal power input to the output port OUT (i.e., CPL signal power/OUT input power [dB]). Directional couplers typically provide an isolation of approximately −15 dB to −30 dB.
Directivity is defined as the ratio of coupling to isolation and is expressed in dB. The higher the directivity, the less the reflected wave power appearing at the coupled port. That is, when the directional coupling has high directivity, substantially only a forward wave component is allowed to appear at the coupled port. The error in the forward power measurement becomes smaller as the directivity increases, as shown in FIG. 18, since the influence of the reflected wave on the wave detecting circuit decreases. This means that it is possible to accurately monitor the forward power by use of the wave detecting circuit even under load variations. That is, the reduction in the reflected wave component of the voltage detected by the wave detecting circuit results in reduced error in the forward power measurement. This prevents the PA from outputting excessive power under load variations, thereby preventing radiation of distorted components.
FIG. 19 shows an exemplary application of power amplifiers and directional couplers. This circuit is often used in cellular phone units with multiband capability, which are growing rapidly in number in recent years. The reference symbols PA1 to PA3 denote power amplifiers designed to operate in different operating bands, and a directional coupler is connected to each of these power amplifiers. This circuit is characterized by the following: the coupled lines (or sub-lines) of the directional couplers connected to the power amplifiers PA1, PA2, and PA3 are connected in series in that order between a terminating resistance of 50 O and an RF-IC (a circuit) including a wave detecting circuit; the isolated port C12 of the coupled line of the directional coupler connected to the power amplifier PA1 is connected to the terminating resistance to terminate the isolated port C12; and the coupled port C31 of the directional coupler connected to the power amplifier PA3 is connected to the RF-IC. Such an interconnection is referred to as a “daisy-chain.”
When implemented on a substrate, this daisy-chain configuration provides simpler circuitry than does a configuration in which the isolated ports C12, C22, and C32 of three directional couplers (such as shown in FIG. 17), respectively, are terminated separately and the coupled ports C11, C21, and C31 of these three directional couplers, respectively, are connected to a switch for selectively connecting one of the coupled ports to the monitoring wave detecting circuit. This daisy-chain configuration is also advantageous in that only one of the power amplifiers (Pas) is operated at one time when the terminal is in operation. That is, the wave detecting circuit monitors the output power of this operating PA. Therefore, in principle no problem is presented even if the output power of each power amplifier is monitored through the coupled lines of the directional couplers for other power amplifiers which are designed to operate in a different operating band than that power amplifier.
Prior art includes Japanese Laid-Open Patent Publication No. 2007-194870 and Japanese Utility Model Laid-Open Patent Publication No. 5-41206 (1993).
FIG. 20 is an equivalent circuit of a directional coupler and components connected thereto. Specifically, this circuit represents a small directional coupler formed on a GaAs substrate together with a power amplifier, and this chip is mounted on a module substrate. FIG. 21 is an exemplary circuit pattern of the circuit shown in FIG. 20. Referring to FIG. 20, the reference symbols IN and OUT denote the input port and the output port, respectively, of the main line 214 of the directional coupler, and CPL and ISO denote the coupled port and the isolated port, respectively, of the coupled line 220 of the directional coupler. The reference symbols Lw1 and Lw2 denote the inductances of the bonding wires connected between the directional coupler on the chip and the module substrate.
FIG. 22 shows the reflection loss in the chip with or without the wires. In FIG. 22, the reference symbol “S33w/o-L, S44w/o-L” represents the reflection loss in the chip without the wires. The reference symbol “S33with-L, S44with-L,” on the other hand, represents the reflection loss in the chip with the wires attached. As shown in FIG. 22, the addition of the wires (which have an inductance) to the chip results in a significant increase in the reflection loss in the chip. This reflection loss increase is approximately 10-15 dB.
The degradation of the reflection loss characteristics of the coupled line as shown in FIG. 22 presents a problem when the coupled lines of several directional couplers are connected in series, as shown in FIG. 19, to achieve multiband capability. That is, referring to FIG. 19, degradation of the reflection loss characteristics of any one of the three coupled lines results in degradation of the combined reflection loss characteristics of the three coupled lines as measured from the RF-IC side. This degradation of the combined reflection loss characteristics will result in manufacturing variations and degradation in the wave detection characteristics of the wave detecting circuit when mounted on the board of the terminal. Therefore, when the coupled lines of several directional couplers are connected in series with one another as shown in FIG. 19 in order to achieve multiband capability, it is necessary to address the problem of degradation of the reflection loss characteristics of the coupled lines.
Thus, when a plurality of coupled lines are connected in series with one another, as in the configuration shown in FIG. 19, it is necessary to improve the reflection loss characteristics of each coupled line over the entire bands in which the power amplifiers PA1 to PA3 operate, as well as to improve the directivity of the directional couplers. However, the bonding wires connected to the chip act to greatly increase the reflection loss in the chip, as described above with reference to FIG. 22. Therefore, it has been difficult to improve the reflection loss characteristics of the coupled lines of directional couplers with inductive connecting elements, such as wires, connected thereto over a wide band.